It has been recently proposed that a particulate filter for capture of particulates in exhaust gas is incorporated in an exhaust pipe, selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen being arranged downstream of the particulate filter, urea water as reducing agent being added between the selective reduction catalyst and the particulate filter, thereby attaining reduction of both the particulates and NOx.
In this case, the addition of the urea water to the selective reduction catalyst is conducted between the particulate filter and the selective reduction catalyst. Thus, in order to ensure sufficient reaction time for thermal decomposition of the urea water added to the exhaust gas into ammonia and carbon dioxide gas requires, it is necessary to prolong a distance between an added position of the urea water and the selective reduction catalyst. However, such arrangement of the particulate filter and the selective reduction catalyst substantially spaced apart from each other will extremely impair the mountability on a vehicle.
In order to overcome this, the inventor invented a compact, exhaust emission control device as shown in FIGS. 1 and 2 and has already filed Japanese patent application No. 2007-29923 directed to the device. In an exhaust emission control device of the prior application, incorporated in an exhaust pipe 4 through which exhaust gas 3 flows from a diesel engine 1 via an exhaust manifold 2 is a particulate filter 5 housed in a casing 7 to capture particulates in the exhaust gas 3; arranged downstream of and in parallel with the particulate filter 5 and housed in a casing 8 is selective reduction catalyst 6 having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. A rear end of the particulate filter 5 is connected to an front end of the selective reduction catalyst 6 through an S-shaped communication passage 9 such that the exhaust gas 3 discharged through the rear end of the particulate filter 5 is passed via forward turnabout into the front end of the neighboring selective reduction catalyst 6.
As shown in FIG. 2 which shows important parts in enlarged scale, the communication passage 9 is the S-shaped structure comprising a gas gathering chamber 9A which encircles the rear end of the particulate filter 5 to gather the exhaust gas 3 through substantially perpendicular turnabout of the gas just discharged from the rear end of the particulate filter by collision of the gas against a wall surface of the gathering chamber, a mixing pipe 9B which extracts forward the exhaust gas 3 gathered by the gathering chamber 9A and which is provided with a urea water addition injector 10 (urea water addition means) midway of the mixing pipe and a gas dispersing chamber 9C which encircles the front end of the selective reduction catalyst 6 such that, through substantially perpendicular turnabout and dispersion of the gas 3 guided forward by the mixing pipe 9B by collision of the gas against a wall surface of the dispersing chamber, the dispersed exhaust gas 3 is introduced into the front end of the selective reduction catalyst 6.
Arranged in front of the particulate filter 5 in the casing 7 is oxidation catalyst 14 for oxidization treatment of unburned fuel in the exhaust gas 3, and arranged behind the selective reduction catalyst 6 in the casing 8 is ammonia reducing catalyst 15 for oxidization treatment of surplus ammonia.
With the exhaust emission control device thus constructed, the particulates in the exhaust gas 3 are captured by the particulate filter 5. Midway of the mixing pipe 9B downstream of the filter, the urea water is added to the exhaust gas 3 by the injector 10 and is thermally decomposed into ammonia and carbon dioxide gas. On the selective reduction catalyst 6, NOx in the exhaust gas 3 is satisfactorily reduced by the ammonia. As a result, both the particulates and NOx in the exhaust gas 3 are reduced.
In this case, the exhaust gas 3 from the rear end of the particulate filter 5 is introduced into the front end of the adjacent selective reduction catalyst 6 through the forward folded communication passage 9, which ensures a long distance between an added position of the urea water midway of the communication passage 9 and the selective reduction catalyst 6 and facilitates mixing of the urea water with the exhaust gas 3 due to fold-back of and thus turbulence of the exhaust gas flow, resulting in ensuring sufficient reaction time for production of ammonia from the urea water.
Moreover, the particulate filter 5 and the selective reduction catalyst 6 are arranged in parallel with each other and the communication passage 9 is arranged along and between the filter 5 and the catalyst 6, so that the whole structure becomes compact in size to substantially improve the mountability on a vehicle.
As a prior art literature pertinent to the invention, there is, for example, the following Patent Literature 1.    [Patent Literature 1] JP 2005-155404A